1. Field of the Invention
The present invention relates in general to Global Satellite System (GSS) receivers, and in particular to a method for synchronizing a radio network using end user radio terminals.
2. Related Art
Cellular telephony, including Personal Communication System (PCS) devices, has become commonplace. The use of such devices to provide voice, data, and other services, such as Internet access, provides many conveniences to cellular systems users. Further, other wireless communications systems, such as two-way paging, trunked radio, Specialized Mobile Radio (SMR) used by first responders, such as police, fire, and paramedic departments, have also become essential for mobile communications.
The Federal Communication Commission (FCC) has implemented a requirement that Mobile Stations (MS), such as cellular telephones be locatable within 50 feet once an emergency call, such as a “911” call (also referred to as “Enhanced 911” or “E911”) is placed by a given cellular telephone. Such position data assists police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need or have legal rights to determine the cellular telephone's position.
Currently, cellular and PCS systems are incorporating Global Positioning Systems (GPS) technology that uses GPS receivers in cellular telephone devices and other wireless transceivers to meet the FCC location requirement.
Such data can be of use for other than E911 calls, and would be very useful for wireless network users, such as cellular and PCS subscribers. For example, GPS data may be used by the MS user to locate other mobile stations, determine the relative location of the mobile station user to other landmarks, obtain directions for the cellular user via internet maps or other GPS mapping techniques, etc.
One significant problem with GPS receivers in a MS is that the GPS receiver may not always have an unobstructed view of the sky causing the received signals to be very weak. Often, the receiver is unable to demodulate the Almanac or Ephemeris data, making it impossible to determine the user's location or accurate GPS time. This problem may be addressed by transmitting Ephemeris and/or Almanac data and GPS time to the receiver over a communication network. A common feature of communication networks is a large and variable transmission delay, making it difficult to transmit accurate (uncertainty less than 1 millisecond) time.
The concept of locating a mobile unit by triangulating a set of ranges from either a set of fixed points (such as cellular transmitters) or mobile transmitters (such as GPS satellites) have a common requirement that the time of transmission is known. This implies that the time at all transmitters must be common, or the differences known. In many systems today, this information is not immediately available since the systems are focused on data transmission rather than ranging. Therefore, there is a need in the art to overcome the problem of transmission delay in both synchronized and unsynchronized networks.
Code Division Multiple Access (CDMA)(TIA/IS-95B) networks use a GPS time reference standard at every base station, and all transmission frames are absolutely synchronized onto GPS time. Therefore, a Mobile Station, by observing particular transitions on frame, master frame or hyper frame, may predict absolute GPS time within tens of microseconds, including radio transmission delay and group delays inside the mobile station or wireless handset.
Other classes of wireless networks, e.g., Time Division Multiple Access (TDMA), GSM, Analog Mobile Phone Systems (AMPS, TACS), DTV, etc., are not synchronized onto GPS time. Still, the accuracy, precision and stability of the master clock used at the base stations is fairly stable, and slowly varies relative to GPS time. Hence, both the time offset and frequency drift are very stable compared to GPS time, and can be monitored at relatively large intervals. However, any timing information derived solely from such a system has limited value, as there is currently no way to derive absolute GPS time from it.
One solution that has been proposed is to locate stationary monitoring entities, called LMU (Local Measurement Units), which are in radio visibility of several base stations (BS) in a given area. The LMU consists of a wireless section and a GPS timing receiver. At intervals, they measure time offset and frequency drift of every base station in the area, relative to GPS time. As one LMU can cover only a few Base Stations, the overlay monitoring network can become quite large and expensive. It necessitates communication links between the LMU's and a central network entity, which logs this information per BS, merges information from different sources (if several LMU's monitor the same Base Station), and delivers this information to a geolocation server if time assistance has to be delivered to a particular MS in the BS's visibility area. This requires several pieces of additional network infrastructure, as well as additional software and maintenance costs for the network operator to enable such a feature. Thus, there is a need in the art to eliminate the need for LMU's and the associated costs.
It can be seen, then, that there is a need in the art for delivering GPS data in a wireless communications systems, including cellular and PCS subscribers, in an efficient manner. It can also be seen that there is a need in the art for GPS capable MS, such as wireless handsets. It can also be seen that there is a need in the art to be able to aid the GPS receiver to speed acquisition and for position determination. It can also be seen that there is a need in the art to be able to aid the GPS receiver to provide more precise position determination. It can also be seen that there is a need in the art for a large cellular system that can use and/or supply GPS information to cellular users for a number of applications, including E911 without the requirement of geographically proximate base stations.